Normal power systems are commercial power systems supplied by electric power companies, and maintenance of power supply quality in commercial power systems is done mainly by an electric power company.
On the other hand, in self-supported power supply systems that are not connected to commercial power systems, maintenance of power supply quality should be made by themselves. Examples of such self-supported power supply systems are power supply systems in vessels and microgrids where plural types of power supplies are combined to form a network in a constant area.
Patent Literature 1 discloses a in-vessel power supply system having a plurality of accessory power generators, a power storage apparatus for performing a charging/discharging operation, and an electric propulsion device is provided with a power detector for detecting an electric power consumed by the electric propulsion device, a starting switch for giving a starting command, and a control circuit for brings the power storage apparatus into a charging mode when a detection signal of the power detector is a threshold value or less and brings the power storage apparatus into a discharging mode according to the starting command of the starting switch so as to make accumulated electric power to be discharged to an in-vessel power bus. Further, in this constitution, when a thrust force of the electric propulsion device is reduced so that a vessel drive forward at a time of vessel entered port and its power consumption is the threshold or less, the control circuit allows the power storage apparatus to be charged from the accessory power generators. Further, Patent Literature 1 discloses that when the starting command is received from the starting switch during the charging operation, the power storage apparatus is allowed to perform the discharging operation so as to supply a discharge power to the electric propulsion device, and an accessory motor for driving the accessory power generator is efficiently operated during a time from the vessel entered port to berthing so that a fuel to be used by the accessory motor and exhaust gas are reduced.
Such an in-vessel power supply system is normally a self-supported power supply system that is independent from a commercial electric power system, and is supplied with electricity in conjunction with the commercial electric power system during berthing and anchorage.
Patent Literature 2 discloses a technique that secures stability and quality of a microgrid using a power converting apparatus provided to an electricity storage facility in the microgrid having a motor power generator, a dispersed power supply using natural energy such as photovoltaic power generation and wind power generation, and an electricity storage facility having a secondary battery.
The in-vessel power supply system and the microgrid constitute a combined power generation system composed of various power supplies such as a power storage apparatus composed of a motor power generator or a secondary battery and a power converter, a power generator using a solar battery, and a power generator using a fuel cell.
Patent Literature 3 discloses a technique relating to a power converting apparatus that is used in a power storage apparatus, and can bear a high-frequency component and an unbalanced component of a load current without additionally providing a detector of a load current. That is to say, the power converting apparatus in Patent Literature 3 includes a controller, and a power converter capable of converting a DC power of a secondary battery into an AC power, and converting the AC power input via an output line into a DC power so as to store it in the secondary battery. The controller is provided with a virtual power generating apparatus instead of the power converter and the secondary battery. The controller includes a virtual power generating apparatus model section for calculating a current value to be output based on a voltage of the output line of the power converter and determining a current command value, and a control signal generator for outputting an electric current corresponding to the current command value to the output line. Patent Literature 3 discloses a technique where in the virtual power generating apparatus model section, an engine model converts a fuel supply quantity to be calculated by a governor model into a mechanical torque of an engine without taking a response characteristic of the engine into consideration so as to calculate an angular velocity and a phase angle of the power generator.
The power converting apparatus has two kinds of apparatuses including a current control type apparatus and a voltage control type apparatus from a viewpoint of its control method. Patent Literature 4 discloses a technique of a current control type power converter. That is to say, it discloses a technique of a parallel operation apparatus constituted so as to suppress a load fluctuation of a self-supported power supply system composed of a power storage apparatus and a synchronous power generator using a DC power supply. Concretely, it discloses a technique that constitutes the parallel operation apparatus where in order to sufficiently cope with an abrupt change in a load and an unbalanced load in the parallel operation apparatus, a signal composed of a negative correction amount due to dropping characteristics of a frequency setting value and a frequency of self-supported power supply system is input into a frequency controller so that an active current setting value is obtained, and a signal composed of a deviation between a voltage setting value and a voltage effective value of the self-supported power supply system and a negative correction amount due to a dropping characteristic of a voltage is input into a voltage controller so that a reactive current setting value is obtained.